2-Hydroxy-4-methylthio-butyric acid (HMTBA) has the formula: ##STR1## It is a known product similar to the essential amino acid DL-methionine whose principal use is a food supplement for feeding animals. In contrast with the amino acid, it is not used directly by the organism in protein synthesis, because it must be anabolically converted into the amino acid to be used a such. HMTBA is not used in the pure form, but in various forms, namely:
(1) concentrates containing a mixture of calcium and ammonium salts of HMTBA, water and calcium sulphate, as in the U.S. Pat. Nos. 2,745,745 and 2,938,053 and in British Patent Nos. 722,024 and 915,193;
(2) acidic aqueous solutions having a concentration of HMTBA of 85-90% by weight, like that described in the U.S. Pat. Nos. 4,353,924 and 3,773,927; and
(3) the calcium salt of HMTBA obtained by the process described in the U.S. Pat. No. 3,175,000.
The general process for obtaining HMTBA from 3-methylthio-propionaldehyde may be summarized by the following scheme: ##STR2## is amply described in the patent bibliography, particularly in the U.S. Pat. Nos. 2,745,745 and 2,938,053 in which step (2).fwdarw.(3) of the above scheme implies passing through the intermediate amide, which leads to HMTBA by a further hydrolysis. However, depending on the conditions of hydrolysis used, the free acid or the intermediate amide is obtained. This hydoolysis reaction of the nitrile is also described in U.S. Pat. Nos. 4,353,924 and 3,175,000 which specifically use sulphuric acid and in U.S. Pat. No. 3,773,927 which uses hydrochloric acid. These patents also describe processing conditions, such as temperature, duration and molar ratios of the inorganic acid to the nitrile. In these patents, once the HMTBA has been obtained by the hydrolysis reaction, it is isolated in the form shown above (salts or aqueous solution) using operations which will be referred to later when comparing the prior art processes with the process of the present invention.
Finally, the European Patent Applications published under Nos. 142,488 and 143,100, both filed on Nov. 13, 1984, particularly relate to obtaining liquid forms of HMTBA by extraction processes with solvents for the HMTBA, from the reaction mixture from hydrolysis of HMTBA carried out in the first case with sulphuric acid and in the second case with an inorganic acid. The two European Patent Applications describe in every detail the conditions which enable solvent extraction to be carried out, as well as the exact nature of the solvents used.
However, these processes of the prior art include various disadvantages derived from the particular manner of carrying out the isolation of the final product HMTBA. These disadvantages can be summarized as follows:
(a) Presence of oligomers of HMTBA with the associated bad odor, color and scarcity of free acid;
(b) Excessive viscosity;
(c) Presence of fine particles of calcium sulphate and/or soluble inorganic salts which render the product impure;
(d) High cost of the final product due to the use of methods such as extraction with solvents; and
(e) Low recovery yields of HMTBA.
These disadvantages are overcome by the process according to the invention. It starts from the mixture from the hydrolysis of HMTBA by sulphuric acid in a single step. This enables an aqueous solution of HMTBA to be obtained without the previous disadvantages, due to the use of a novel process for isolating HMTBA in the form of an aqueous solution.
The above references are hereby incorporated by reference.